Acute myeloid leukemia (AML) is a hematologic malignancy characterized by the proliferation of a transformed clone of myeloid progenitor cells. Juxtaposition of the AML1 gene on chromosome 21 to the ETO gene on chromosome 8 fuses the amino-terminal portion of AML1 with near-full length ETO, creating the AML1/ETO chimeric fusion. (Miyoshi et al., Proc. Natl. Acad. Sci. USA 88: 10431-10434 (1991); Erickson et al., Blood 80:1825-1831 (1992); Miyoshi et al., EMBO J. 12: 2715-2721 (1993)). AML1 upregulates a number of target genes critical to normal hematopoiesis, whereas the AML1/ETO fusion interferes with this trans-activation.
The t(8;21)(q22;q22) translocation, associated with AML with maturation (M2 morphology), is one of the most frequently-observed nonrandom genetic alterations. (Look, A. T., Science 278: 1059-1964 (1997)). The portion of AML1 contained in the fusion includes a central 118 amino-acid domain homologous to the Drosophila segmentation gene runt (Erickson et al., Blood 80:1825-1831 (1992)), which serves to bind the enhancer core DNA sequence TGT/cGGT. (Kagoshima et al., Trends Genet. 9: 338-341 (1993)). AML1 is able to form a heterodimer with core-binding factor β (CBFβ). The AML1-CBFβ transcription factor is an important regulator of a number of target genes involved in hematopoiesis, many of which are homeobox-containing HOX genes. (Look, A. T., Science 278: 1059-1964 (1997), Wang et al., Cell 87: 697-708 (1996)). Murine embryos with targeted mutations in AML1 lacked fetal liver hematopoiesis, reinforcing the notion that AML1 is critical to normal blood cell development. (Okuda et al., Cell 84: 321-330 (1996)).
The AML1/ETO fusion retains the ability to interact with the enhancer core DNA sequence via the runt homology domain (RHD) and interferes with the expression of AML1-responsive target genes. (Meyers et al., Mol. Cell. Biol. 15: 1974-1982 (1995); Frank et al., Oncogene 11: 2667-2674 (1995)). In mice heterozygous for a “knocked-in” AML1/ETO allele, hematopolesis was profoundly impaired (Yergeau et al., Nature Gen. 15: 303-306 (1997)) as in the AML1 knock-out mice (Okuda et al., Cell 84: 321-330 (1996)), providing evidence that the chimeric fusion blocks wild-type AML1 function in a transdominant manner. The AML1/ETO fusion contains nearly full-length ETO, missing only a small region with no DNA-binding or transcription regulation motifs. ETO is a phosphoprotein that is normally expressed in brain tissue (Miyoshi et al., EMBO J. 12: 2715-2721 (1993)) and in CD34+ hematopoietic cells. (Erickson et al., Blood 88: 1813-1823 (1996)). Ectopic expression of ETO in NIH3T3 cells, however, leads to transformation. (Wang et al., Cancer Res. 57: 2951-2955 (1997)). With two zinc finger motifs and proline-rich or proline/serine/threonine-rich regions, ETO structurally resembles a transcription factor (Miyoshi et al., EMBO J. 12: 2715-2721 (1993); Erickson et al., Cancer Res. 54: 1782-1786 (1994)), although DNA-binding properties have not yet been confirmed. Mutation analysis has identified ETO sequences within the chimeric fusion as being required for the dominant repression of transcription of AML1 target genes. (Lenny et al., Oncogene 11: 1761-1769 (1995)).
Recently, other onco-regulatory proteins involved in transcriptional repression have been found to interact with co-repressor factors that subserve important functions in modifying chromatin structure by histone deacetylation. (Heinzel et al., Nature 387: 43-48 (1997); Alland et al., Nature 387: 49-55 (1997)). Mad and Mxi1 proteins are antagonists of the Myc family of transcription factors. Mxi1-mediated inhibition of Myc requires interaction with mammalian Sin3 (mSin3A or mSin3B) proteins. (Alland et al., Nature 387: 49-55 (1997)). The nuclear receptor co-repressor (N-CoR) and histone deacetylase (HDAC1) are two other members of a resultant complex that represses transcription by enzymatic deacetylation of histones and creation of a repressive chromatin structure. (Heinzel et al., Nature 387: 43-48 (1997); Alland et al., Nature 387: 49-55 (1997)). The understanding of human proteins involved in the modulation of transcriptional repressor complexes remains in its infancy.